In recent years, a sealed secondary battery (which may be hereinafter simply referred to as “secondary battery”) represented by a lithium ion secondary battery is used as a power source not only for a mobile apparatus such as a portable phone or a notebook personal computer but also for an electrically driven vehicle such as an electric automobile or a hybrid car. A cell constituting the secondary battery has a structure in which an electrode group is accommodated in an inside of a sealed outer casing, and the electrode group is constructed by winding or stacking a positive electrode and a negative electrode with a separator interposed therebetween. Generally, a laminate film such as an aluminum laminate foil or a metal can having a cylindrical shape or a prismatic shape is used as the outer casing.
Such a secondary battery has a problem that, when the electrolytic solution is decomposed due to overcharging or the like, the cell swells to be deformed in accordance with rise in the internal pressure caused by the decomposition gas and, unless the charging current or the discharging current is stopped, this results in rupture of the secondary battery. In order to cope with this problem, a safety valve is conventionally provided that is opened when an internal pressure comes to have a predetermined value or more (for example, about 1 to 2 MPa); however, this has another problem because the electrolytic solution is scattered to contaminate the surroundings when the safety valve is opened. Therefore, it is desirable to monitor an inside state of the cell so that the rupture can be prevented even if the safety valve is not opened. For that purpose, there is a need for a technique capable of detecting, with a high degree of sensitivity, the internal pressure change of the cell or the electrode swelling (dilation deformation of the electrode group) accompanying the charging and discharging of the cell.
Patent Document 1 discloses a monitoring apparatus in which a pressure sensor is disposed in an inside space of a safety valve of a lithium secondary battery, and the pressure detected by the sensor is displayed on a displaying device. However, this necessitates an electric wire from the pressure sensor located in the inside of the container to the displaying device located on the outside of the container. Therefore, unless a structure for sealing the surroundings of the electric wire is added, the sealed structure is hindered. Also, Patent Document 2 discloses a sealed storage battery including a pressure-sensitive electroconductive rubber that changes a resistance value thereof in accordance with rise in the internal pressure provided in a battery case. However, an electric wire from the inside to the outside of the battery case is provided, and a special structure for retaining the sealed structure is needed in the battery case.
Patent Document 3 discloses a laminate-type battery in which, in a part of a welded part in which the fringes of a laminate film are welded to each other, a part in which metal layers are in contact with each other without intervention of a resin layer is formed, and rise in the internal pressure is detected by change in the voltage value or change in the resistance value of the metal layer when that part is peeled off. However, in this battery, detection is not carried out unless the internal pressure becomes so high that the welded part is peeled off. Also, forming the part, in which the resin layer is absent or the metal layer is exposed at the welded part can disadvantageously be a cause of breakdown.